1. Field of the Invention
Embodiments of the present invention generally relate to solar/photovoltaic cells and the method of forming selective emitters for the same.
2. Description of the Related Art
Solar or photovoltaic cells are material junction devices which convert sunlight into direct current (DC) electrical power. When exposed to sunlight (consisting of energy from photons), the electric field of solar cell p-n junction separates pairs of free electrons and holes, thus generating a photo-voltage. A circuit from p-side to n-side allows the flow of electrons when the solar cell is connected to an electrical load, while the area and other parameters of the SOLAR cell junction device determine the available current.
Currently, solar cells and panels are manufactured by starting with many small silicon sheets or wafers as material units and processing them into individual solar cells before they are assembled into modules and panels. These silicon sheets are generally saw-cut p-type boron doped silicon sheets, precut to the sizes and dimensions that will be used. The cutting (sawing) or ribbon formation operation on the silicon sheets damages the surfaces of the precut silicon sheets to some degree, and etching processes are performed on both surfaces of the silicon sheets to remove a thin layer of material from each surface and provide textures thereon.
P-N junctions, a critical component of emitters, are then formed by diffusing or implanting an n-type dopant into the precut p-type silicon substrate. Phosphorus is widely used as the n-type dopant for silicon in solar cells. One example of phosphorus diffusion process includes coating phosphosilicate glass compounds onto the surface of the silicon sheets and performing diffusion/annealing inside a furnace. Another example includes bubbling nitrogen gas through liquid phosphorus oxychloride (POCl3) sources which are injected into an enclosed quartz furnace loaded with batch-type quartz boats containing the silicon sheets.
Following emitter formation, one or both surfaces of the solar cell can also be coated with suitable dielectrics. Dielectric layers are used to minimize surface charge carrier recombination and some dielectric materials, such as silicon oxide, titanium oxide, or silicon nitride, can be provided as antireflective coating to reduce reflection losses of photons.
The front or sun facing side of the solar cell is then covered with an area-minimized metallic contact grid for transporting current and minimizing current losses due to resistance through silicon-containing layers. Some blockage of sunlight or photons by the contact grid is unavoidable but can be minimized. The bottom of the solar cell is generally covered with a back metal which provides contact for good conduction as well as high reflectivity. Metal grids with patterns of conductive metal lines are used to collect current. Generally, screen printing thick-film technology is used in the solar cell industry to layer a conductive paste of metal materials, e.g., silver, etc., into a desired pattern and deposit a metal material layer to the surface of the silicon sheets or substrates for forming metal contact fingers or wiring channels on the front and/or back side of the solar cell. Other thin film technologies may be used for contact formation or electrode processing. The deposited metal layer, formed into contacts, is often dried and then fired or sintered at high temperature to form into good conductors in direct contact with underlying silicon materials, and a single solar cell is made. Generally, both silver and aluminum are contained in the screen printing paste for forming back side contacts with good conductor contact to silicon material and easy soldering.
Manufacturing high efficiency solar cells at low cost (providing low unit cost per Watt) is the key to making solar cells more competitive in the generation of electricity for mass consumption. Even small improvements in cost per Watt substantially increase the size of the available market. Therefore, there exists a need for a cost effective method of forming emitters to improve the efficiency of a solar cell in generating and maintaining electron-hole pairs from absorbed photons in the emitters and the efficiency of driving the electrons and holes through the external electrical circuit with a load.